Wet strength is a desirable attribute of many disposable paper products, such as napkins, paper towels, household tissues, disposable hospital wear, etc., which come into contact with water in use. However, an untreated cellulose fiber assemblage will typically lose 95% to 97% of its strength when saturated with water. To improve the wet strength paper manufacturers have for years been adding chemicals during the paper making process.
These wet strength additives result in a permanent wet strength, i.e., paper which when placed in an aqueous medium retains its initial wet strength over time. Permanent wet strength in the paper products is often an unnecessary and undesirable property. Paper products such as toilet tissues, etc., are generally disposed of after brief periods (within about 5 minutes) of use into septic systems and the like. Clogging of these systems can result if the paper product permanently retains its hydrolysis-resistant strength properties.
Consequently there is a need to develop a wet strength resin that will produce a paper product with good initial wet strength and which retains sufficient wet strength for its intended use, but when disposed of in a septic system has lost sufficient wet tensile strength so as to clear the septic system.
In spite of the successful use of chemicals to increase permanent wet strength, the mechanism by which wet strength additives work is not totally understood. See Stannet "Mechanisms of Wet Strength Development in Paper" Surface Coatings Related Paper Wood Symp. pp. 289-99 (1967) and Westfelt, "Chemistry of Paper Wet Strength. I. A Survey of Mechanisms of Wet Strength Development" Cellulose Chemistry and Technology Vol. 13, pp. 813-825 (1979).
Cellulose fibers have adequate strength when dry; this dry strength is thought to result from strong interfiber hydrogen bonding. When the fibers are wetted, water will penetrate the fibers, disrupt the internally hydrogen bonded network and, as a consequence, lower the strength of the paper. Therefore, any approach to improve the wet strength of paper must either prevent water from reaching and disrupting internal hydrogen bonds or allow for the formation of new bonds incapable of being broken by water. Both approaches are being practiced in the art, but it is the latter which has been the historical method of choice.
In this latter method a relatively low molecular weight, water soluble resin (wet strength agent) is added to the pulp before the paper product is formed (wet-end addition). In order for the resin to provide wet strength in the finished product, two events must then occur. First, the resin must be retained by the fiber and secondly there must be formation of new chemical bonds. Since cellulose always carries some negative charge due to the presence of carboxylic acid groups, the resin is frequently designed to be cationic.
The most common wet strength resins include the urea-formaldehyde and melamine-formaldehyde type resins. These types of resins require curing under acidic conditions. Curing can be accomplished by storing paper which has been treated with a urea-formaldehyde resin at ambient temperatures for a period of days during which time the wet strength develops, or by the application of heat which can accelerate the cure rate to a matter of minutes. However, due to this requirement of acidic conditions (pH of about 4 or 5) for curing, corrosion of the papermaking equipment and premature embrittlement of the paper frequently occur. Alternative resins such as polyamide-epichlorohydrin wet strength resins can be used. Yet paper utilizing these resins also requires an aging period at ambient temperature. Again, curing can be accelerated by the addition of heat. However, storage and/or the addition of heat to the paper product poses a serious handicap to its rapid production, as well as increasing its cost of production.
To avoid these problems, wet strength resins formed by reacting glyoxal (CHOCHO) with water-soluble vinylamide polymers have been suggested by U.S. Pat. No. 3,556,932 to Coscia et al., issued Jan. 19, 1971. Papers incorporating such resins lose only about half their wet strength upon exposure to water. Paper products with this level of wet tensile decay could cause substantial septic system clogging. U.S. Pat. No. 3,709,857 to Faessinger, issued Jan. 9, 1973, suggests reacting glyoxal with acrylamide and then polymerizing the glyoxalated acrylamide with acrylamide in the presence of a basic nitrogen-containing monomer. Unfortunately, paper products made according to these methods also have too much permanent wet strength.
As discussed above, permanent wet strength is often unnecessary and undesirable. The temporary wet strength resin of the present invention has a good initial wet strength and loses sufficient wet tensile strength to clear a septic system without clogging. This is a property that is not fulfilled by the urea-formaldehyde, polyamide-epichlorohydrin or glyoxylated acrylamide resins described above. None of these resins lose much more than 50% of their initial wet tensile even after prolonged exposure to aqueous solution. U.S. Pat. No. 3,096,228 to Day et al., issued July 2, 1983, describes paper products wherein a solution of glyoxal is applied to a paper web. Upon exposure to water, this paper is able to resist a rupture for only about a minute and disintegrates within about 3 minutes, therefore not providing sufficient wet strength for any appreciable length of time. Additionally, glyoxal cannot be added to the wet-end of the papermaking process because glyoxal carries no cationic charge and is therefore not substantive to cellulosic fibers.
U.S. Pat. No. 3,740,391 to Williams et al., issued June 19, 1973, describes a water-soluble thermosetting wet strengthening agent for paper which is prepared by reacting an amidated ionic glucopyranosyl compound with glyoxal. Paper products containing this wet strength resin lose about one half their original wet strength during 24 hours of normal wet weathering. Such paper products are not suited to use as toilet tissues and the like since their wet strength degradation is far too slow. This is unacceptable for normal septic systems.
It is therefore an object of this invention to provide paper products which can be easily disposed of in normal septic systems.
It is a further object of this invention to produce paper products which lose their wet tensile strength over an appropriate period of time in an aqueous medium.
It is still a further object to provide effective temporary wet strength resins which do not adversely affect softness and absorbency when incorporated into paper products.
These and other objects of this invention will become apparent by the description of this invention below.